1. Field of the Invention
The present invention relates to an ESD protection device. In particular, the present invention relates to technologies for preventing breakdown and deformation of a ceramic multilayer substrate caused by, for example, cracking in an ESD protection device that includes discharge electrodes facing each other in a cavity of the ceramic multilayer substrate.
2. Description of the Related Art
ESD (electro-static discharge) is a phenomenon in which strong discharge is generated when a charged conductive body (e.g., human body) comes into contact with or comes sufficiently close to another conductive body (e.g., electronic device). ESD causes damage or malfunctioning of electronic devices. To prevent it, it is necessary not to apply an excessively high discharge voltage generated during discharge to circuits of the electronic devices. ESD protection devices, which are also called surge absorbers, are used for such an application.
An ESD protection device is disposed, for instance, between a signal line and ground of the circuit. The ESD protection device includes a pair of discharge electrodes facing each other with a space disposed therebetween. Therefore, the ESD protection device has high resistance under normal operation and a signal is not sent to the ground. An excessively high voltage, for example, generated by static electricity through an antenna of a mobile phone or the like causes discharge between the discharge electrodes of the ESD protection device, which leads the static electricity to the ground. Thus, a voltage generated by static electricity is not applied to the circuits disposed downstream from the ESD protection device, which allows for protection of the circuits.
An ESD protection device shown in an exploded perspective view of FIG. 9 and a sectional view of FIG. 10 includes a cavity 5 formed in a ceramic multilayer substrate 7 made by laminating insulating ceramic sheets 2. Discharge electrodes 6 facing each other and connected to external electrodes 1 are disposed in the cavity 5 that contains a discharge gas. When a breakdown voltage is applied between the discharge electrodes 6, discharge is caused between the discharge electrodes 6 in the cavity 5, which leads an excessive voltage to the ground. Consequently, the circuits disposed downstream from the ESD protection device can be protected (for example, refer to Japanese Unexamined Patent Application Publication No. 2001-43954).
However, such an ESD protection device has the following problem.
In the ESD protection device shown in FIGS. 9 and 10, the responsiveness to ESD easily varies due to the variation in the space between the discharge electrodes. Furthermore, although the responsiveness to ESD needs to be adjusted using an area of the region sandwiched between discharge electrodes facing each other, the adjustment is limited because of a product size or the like. Therefore, it may be difficult to achieve a desired responsiveness to ESD.